Quantum well devices have been developed recently to provide classical optical communication system functions such as modulation, detection, and optical signal generation. See, for example, U.S. Pat. No. 4,525,687. By employing the nonlinear bistability of certain devices, it has been possible to extend the growth of quantum well devices into the areas of switching systems and optical computing. In the latter areas, quantum well devices have been fabricated as basic Boolean logic devices such as AND and OR gates, complex memory and processing devices such as S-R flip-flops and logic device arrays for parallel processing, and switching devices such as n x n switching arrays.
Improvements to the latter quantum well devices have centered on the development of the self electrooptic effect device, more commonly referenced as the SEED device. See U.S. Pat. No. 4,546,244. In the operation of the SEED device, a photocurent generated from the absorption of optical energy by a diode may change the voltage across the diode which, in turn, causes a change in the absorption characteristic of the diode. The diode is generally a GaAs/AlGaAs p-i-n structure wherein the intrinsic region (i) comprises one or more quantum well layers. Connection of the diode to a proper electrical load such as a resistive load and tuning the optical signal wavelength coincidently with the heavy hole exciton resonance wavelength permits switching and bistable behavior (hysteretic operation) to be achieved via a positive feedback mechanism.
An integrated structure for the SEED device has been proposed in which a p-i-n diode having quantum wells in the intrinsic region is vertically integrated in series with a standard photodiode that is used as a "resistive" load. See D. A. B. Miller et al., Appl. Phys. Lett., Vol. 49, No. 13, pp. 821-3 (1986). The two diodes are responsive to different wavelengths such that the load photodiode is transparent to the infrared wavelengths used in the quantum wells of the other diode and is opaque to wavelengths shorter than infrared. When operated, a control light beam shone on the load photodiode effectively varies the resistance of the load in the circuit thereby affecting the switching characteristics (speed and power) of the integrated SEED device. In addition to the control light beam, an input infrared light beam is directed on the SEED device and a single infrared light beam is output from the device.
In another related development, a symmetric SEED device has been proposed in which two serially connected p-i-n photodiodes are fabricated with substantially identical quantum well layers in the respective intrinsic regions. See A.Lentine et al., Conference on Lasers and Electrooptics, Paper ThT12 (1987). This structure is called symmetric because each photodiode operates as the load for the other. It has been proposed that this structure act as a bistable optical memory element such as an optical S-R flip-flop. As an S-R flip-flop, the structure supports dual inputs and dual outputs wherein the optical wavelengths for all inputs and outputs are identical.
It should be clear to those skilled in the art that none of the devices described above are capable of receiving dual input optical signals at different wavelengths and, in response to the dual input signals, generating dual output signals at the corresponding different wavelengths wherein at least one output signal is functionally related to the input signal of the different wavelength. For example, it is not possible for information borne by an optical signal at a first wavelength to be transferred intact to another optical signal at a second wavelength and vice versa via any optoelectronic circuit employing quantum well devices as described above. In other words, the devices described above are not capable of bidirectional information transfer with bidirectional wavelength conversion.